Updated SimulateGrid + tests + added as_MltplxExperiment generic

[nateosher]

I updated a few major things here:

• I updated SimulateGrid to return a MltplxObject rather than a ppp object- this felt more consistent. It also now generates a random slide id.
• I added a generic, as_MltplxExperiment, for converting other objects to MltplxExperiments. Right now I've only implemented it for a list consisting of MltplxObjects, but this is probably a good jumping off point for converting other things
• Obviously this broke a bunch of tests and required some new ones, so I took care of that too

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.17 :tada:

Comparison is base (bfcf8e7) 95.21% compared to head (e7c338f) 95.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #85 +/- ## ========================================== + Coverage 95.21% 95.38% +0.17% ========================================== Files 32 32 Lines 898 932 +34 ========================================== + Hits 855 889 +34 Misses 43 43 ``` | [Impacted Files](https://codecov.io/gh/nateosher/DIMPLE/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [R/MltplxExperiment.R](https://codecov.io/gh/nateosher/DIMPLE/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9NbHRwbHhFeHBlcmltZW50LlI=) | `93.79% <100.00%> (+0.69%)` | :arrow_up: | | [R/MltplxImage.R](https://codecov.io/gh/nateosher/DIMPLE/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9NbHRwbHhJbWFnZS5S) | `100.00% <100.00%> (ø)` | | | [R/MltplxObject.R](https://codecov.io/gh/nateosher/DIMPLE/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9NbHRwbHhPYmplY3QuUg==) | `97.29% <100.00%> (ø)` | | | [R/SimulateGrid.R](https://codecov.io/gh/nateosher/DIMPLE/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9TaW11bGF0ZUdyaWQuUg==) | `100.00% <100.00%> (ø)` | | | [R/generics.R](https://codecov.io/gh/nateosher/DIMPLE/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9nZW5lcmljcy5S) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[mariamasotti1]

this looks good I fixed one typo and committed it